Amplifying system



c. H. RUMPEL AMPLIFYING SYSTEM Fiied Dec. 24, 1940 Feb. 29, 1944.

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7NVEN70R c. H. RUMPEL WHJHM? ATTORNEY 3.000 FREQUENCY IIV CVCLES PERSECOND m 4M 1 1 2 1 I 9 0 I 1 5 I m Q m M H m p M a urn D D S E M E S EE5 L E r F m" m b xv M 2 lc M -4 HI N A. MMYI G. H G c E m F mm H mm -mW T U T A n u F m "u M NO o L. o .M. 0 c w A u o l l 2 Z 3 9 6 6 5 4Patented Feb. 29, 1944 AMPLIFYING SYSTEM Carl H. Rumpel, JacksonHeights, N. Y., assignor to Bell Telephone Laboratories Incorporated,

New York, N. Y., a corporation of New York Application December 24,1940, Serial No. 371,498

Claims. (Cl. 179-107) This invention relates to amplifying systems andthe object of the invention is an improved amplifier for use in portableapparatus such as audiphones.

In all battery operated amplifiers, particularly v those with more thanone stage, the amplifier gain changes due to the internal resistance ofthe plate battery increasing as the battery becomes discharged. This iscaused by the signal voltages developed across the internal resistanceof the battery being fed back both regeneratively and degeneratively toprior stages. The magnitude of these feedback voltages may easily besufilcient to cause the amplifier to oscillate in the case of excessregenerative feedback or for'the gain to be reduced appreciably due todegenerative feedback. In the case of an audiphone amplifier, theseeffects are particularly troublesome because the weight limitation forsuch apparatus requires the use of miniature, short-lived batteries anddoes not permit connecting large capacity condensers across the platebattery to by-pass the feedback voltages.

Aside from the gain limitations of small portable amplifiers, audiphoneusers frequently experience difficulties in hearing intelligibly innoisy locations since the noise and the low frequency sound componentsoverload the system and produce excessive distortion.

In accordance with an important feature of this invention, the volumelevel of the output is controlled by variable negative feedback so thatfor the lower sensitivities the feedback is greater thereby givingbetter quality of reproduction for those who have only slightly impairedhearing and who therefore are more likely to appreciate the betterquality.

According to another feature of the invention,

'a circuit is provided for introducing a positive feedback proportionalto the internal resistance of the battery so that the gain of theamplifier is substantially independent of the battery resistance or may,if desired, be made to increase or to decrease slightly as the batteryis expended.

The pick-up microphone is connected to the amplifier through anadjustable network which gives the user a choice of several lowfrequency transmission characteristics according to his personalrequirements or the particular characteristics of the sounds he wishesto hear. In noisy locations this network may be set to give a largeamount of low frequency attenuation, thereby discriminating against lowpitched noises and the lower sound frequencies of the voice whichcontribute but little to intelligibility. This procedure permits ofgreater amplification of the higher frequencies without overloading.

These and other features of the invention will be more clearlyunderstood from the following detailed description and the accompanyingdrawing, in which:

Fig. 1 is a circuit diagram of an amplifying system according to theinvention, using a crystal type receiver;

Fig. 1A shows an electromagnetic receiver adapted for use with thecircuit of Fig. 1;

Fig. 2 shows the frequency response curves of the system;

Figs. 3, 4 and 5 show the elements of the coupling network connected invarious ways to produce the several different low frequency responses;and,

Fig. 6 is a series of curves illustrating the use of feedback to controlthe volume and maintain a fiat response and high gain independently ofthe aid of the battery.

Referring now to Fig. 1, the pick-up microphone II is preferably of thepiezoelectric crystal type which, as is well known, is a high impedancedevice and may be connected directly to an amplifier without the use ofa transformer. In this case, the microphone is coupled to the grid ofthe tube It by means of the adjustable network 13, the function of whichwill be described more in detail below. sistance-capacity coupled andthe output transformer It; connects the tube IE to the magnetic typereceiver I! of Fig. 1A in the conventional manner, or, alternatively, tothe crystal receiver 44 in a novel manner to be described.

The battery l8 provides filament current for all three tubes and thebattery I9 supplies the required voltages to the screen grids and platesthrough suitable resistors. The primary of the output transformer isshunted by the potentiometer 20 by means of which an adjustable portionof the output energy may be fed back negatively .to the prior stages.path including conductor 2|, resistor 22, the condenser 23 and theresistor 24 will produce signal voltages across the latter resistorwhich are out of phase with the signal voltages from the tube It andtherefore reduce the gain and correspondingly decrease the distortion.The resistor 22 also passes plate current for the tube ll, therebymaking it unnecessary to provide a separate resistive connection to thebattery for this purpose.

Another portion of the output signal energy flows through resistors 25and 26 and condenser The tubes I2, I! and i5 are re- Current flowingover the 21 to the negative side of the battery thereby impressingvariable signal potentials on the screen grid 28 of the tube l2. Thesepotentials are also in negative sense and therefore reduce the gain anddistortion in all three stages. The resistor 25 and the conductor 2!also provide a direct current path for the screen current of the tubel2.

The elimination of the usual coupling condenser from the feedback pathis particularly advantageous in portable apparatus such as audiphoneswhere a bulk and weight should be kept as small as possible for, ifused, such a condenser would have to be of large capacity to keep theamplifier stable at low frequencies.

As the battery resistance increases with use, the signal potentialsdeveloped across the battery terminals become appreciable and introduceother feedback at various points in the circuit,

.the most important of which are at the screen grid 28 of the tube i 2and the control grid of the tube H. The feedback to the screen grid 28will be negative, thereby reducing the gain of the amplifier even whenthe potentiometer is at its lowest or zero feedback setting. Thefeedback potentials are impressed on the control grid of the tube isover a circuit including resistors 30 and 33 and the coupling condenser34 in a postitive or regenerative sense. This positive feedback wouldordinarily be so much greater than the negative feedback to the screengrid 28 as to make the amplifier unstable but it is largely eliminatedby the conventional filtering elements, condenser 29 and resistance IIIand condenser 3i The resistance Ill and condenser 2i, however, are noteifective in reducing the negative feedback to the screen grid 28 andhence this feedback accentuates the normal loss of gain as the batteryages. To compensate for this undesirable effect, resistor 321s connectedin series with the filter condenser 3! across the plate circuit of thetube I2. The signal potentials existing across the battery resistancewill build up across resistor 32 proportional potentials which areapplied to the grid of the tube ll through the condenser 34 as apositive feedback which is a function of the battery resistance. Byproper choice of the value of resistor 32, this feedback may be madeequal to, greater than or less than the loss of gain due to the aging ofthe battery,

as desired. While an amplifier gain which is independent of batteryresistance is very desirable, in practice it may be preferable to useslightly less positive feedback and permit the gain to decrease veryslightly as the battery ages in order to avoid the possibility of everobtaining enough over-compensation to make the system unstable.

The network i3, referred to above as providing the user with a choice oflow frequency response characteristics, comprises a condenser 35,resistors as to 39 and a three-position switch I. The problem ofobtaining a high rate of attenuation of low frequencies in the input ofthis system is complicated by the very high impedance of the crystalmicrophone and the necessity for keeping the bulk and weight of thefilter elements very small. Conventional inductance and capacitynetworks are entirely impractical since they are too large and heavy andmany case, to be eflective, they would have to be placed in the lowimpedance output circuit where they would not protect the tubes againstoverloading. On the other hand, the more usual, simpleresistance-capacity networks are unsuitable since when designed for therequired low frequency attenuation they also attenuate the middlefrequencies excessively.

In order to obtain the required high attenuation at low frequenciesonly, with resistancecapacity elements in the input circuit, the networkl3 has been designed as a two section filter with the transmitter llserving as the capacity rate of cut-off approximating twice that of asingle section is obtained. The two-section filter giving the highattenuation of low frequencies'shown by curve ll of Fig. 2 is used whenthe switch 40 is in position I, in which case the network is effectiveas shown in Fig. 3.

When the switch is moved to position 2, the shunt impedance to groundthrough resistors 26 and I1 is increased by connecting in resistor 32 asshown in Fig. 4 thereby reducing the attenuation and giving the lowfrequency characteristic of curve 42. In position 3 the switch is outout of circuit and the shunt impedances are greatly increased by theaddition of the very high resistor 29 which, as shown in Fig. 5,virtually disconnects resistors 36 and 31 from ground so that theybecome merely a by-pass around the condenser 35. The condenser istherefore effectively removed from the circuit and the response becomesessentially flat over the usable portions of the frequency range, asshown by curve 42.

It will be observed from the curves of Fig. 2 that the filter networkunder the control of the simple single point switch ll gives a choice ofwidely different low frequency characteristics without producing anyappreciable change in the response of the system for the higherfrequencies which are important from an intelligibility standpoint. Inone very satisfactory network of this type the capacities of themicrophone II and condenser 35 are 750 f and 250p f respectively,resistors 30 and II are each .250 resistor 31 is 0.60 and resistor II is109. These values are, of course, merely illustrative and may be ofother values depending on the particular transmitter and tubes used andon the degree of attenuation required.

The system is designed to be used with either a magnetic type receiverI! or with a crystal receiver 44. When the magnetic receiver is used thecontacts 45, u of the plug 41 are connected to the output terminals 48,ll of the transformer II in the conventional manner. If desired,protection of the user from acoustic shock may be obtained by using aplug having a third contact III which connects one side of a limitingdevice 52 through terminal Ii and condenser ll to the plate terminal ofthe primary winding of the transformer. The other side of the limitingdevice is connected to the negative terminal of the plate battery sothat the device is effectively in shunt to the output of tube II. Thislimiting device may be of any known type such, for example, as anon-linear resistor of the substanceknown as thyrite. the resistance ofwhich decreases rapidly above a certain critical voltage. In cases wherea more gradual limiting some other known form of limiter such as athermal device the resistance of which varies with its temperature. 1The crystal receiver 44 is electrically equivalentto a small capacity ofthe order of v 750ml! and its' impedance over the frequency range of 100cycles to 10,000 cycles per second therefore varies from about 20,000ohms to 2 megohms. As is well understood in the art an outputtransformer suitable for use with a magnetic receiver is thereforeimpractical for use with a crystal receiver.

It has been shown that a constant K bandpass filter with a full shuntarm at the input will develop the maximum voltage across a condenserwhich is uniform -over the frequency band. While such a network could beused to couple the crystal receiver to the amplifier this,

would involve a number of additional circuit elements which would notonly increase the cost of the system materially but it would also addconsiderably to the bulk and weight of the system which would be aserious objection in the case of portable apparatus such as audiphones.

Applicant has found, however, that the transformers necessary forcoupling the magnetic receiver to the amplifier may be used with thecrystal receiver in such a manner as to simulate the eii'ect of aconstant K network. In this case a terminating resistance is connectedacross the secondary of the transformer and'the crystal receiver isconnected across the amplifier output through a suitable blockingcondenser. For best results the transformer should have a very highleakage reactance but since this would ordinarily require a transformerof considerable bulk there are practical limitations on the leakagereactance in cases where the bulk and weight must be kept small. Whilethe value of the terminating resistance will depend on the con stants ofthe particular transformer used and other factors the optimum value canbe readily determined by experiment. In one instance, for example, avery ood response characteristic was obtained with a very smalltransformer suitable for use with the magnetic receiver with a secondaryshunting resistance of 60 ohms.

In the circuit shown, when the plug 6| is inserted in the output jackone terminal of the crystal receiver is connected through contact SI andthe blocking condenser 53 to the plate of the output tube l and theother terminal is connected through contact 48 to the cathode of thetube. At the same time a resistor 54 of the proper value is connectedacross the secondary of the transformer through contact 48 and thesleeve 62 of the jack.

The shunting resistor 54 is, of course, not required when the magneticreceiver is used and because of the inherently limited output capacityof the crystal receiver, the voltage limiter is not required when thecrystal is used. The resistor and limiter are, therefore, associatedwith the individual receivers and are preferably built into therespective receiver plugs as indicated so that the necessary circuitchanges are made automatically when either receiver is connected foruse.

Since the small crystal audiphone receiver is essentially a capacitativeimpedance a transformer of high leakage reactance may be used in themanner described to apply a constant voltage to other capacitative loadssuch, for example, as crystal or capacity type loud-speakers.

In cases where good low frequency response is important a slightlybetter characteristic may be. obtained by connecting a suitablecondenser in series with the resistor 54.

The advantages of this amplifier from the standpoint of high quality andsustained gain over the useful life of the battery are illustrated inFig. 6. As shown by curve 65 whenv the batteries are new'and theamplifier is operating at maximum gain, that is,- with no negativefeedback, the response is'reasonably uniform from 100 to 10,000 cycles.This very large gain is very rarely, if ever, required, however, and asshown by curve 56, the response becomes much more nearly uniform whenthe output volume is decreased by only 5 decibels of negative feedback.The response characteristic is, of course, still further improved asmore feedback is used and with 25 decibels feedback there is noperceptible variation of gain as shown by curve 58 but the output levelis still ample for a large proportion of audiphone users.

Curve 5'! shows the general configuration of the response curve formaximum gain when the batteries have deterioratedto a point where theirvoltage is only two-thirds of the new battery voltage. While this curveshows some reduction of low frequency response, this loss of gain isvery much less than in prior audiphone I amplifiers and the circuitshows no tendency to oscillate. It will also be understood that whilecurve 51 shows an appreciable loss of gain as compared with the gainwhen using new batteriespthis is largely a matter of choice since it isquite feasible to use enough positive feedback to keep the gainpractically constant over the whole range until the batteries arecompletely unfit for further use.

But even when the positive feedback used is not sufficient in itself tomake the gain independent of the battery resistance at maximum gain, thegain and the uniformity of response with maximum negative feedback areaffected only slightly by the increasing battery resistance as shown bycurves 59 and 60.

While the various features of the invention have been described withparticular reference to audiphones, it will be apparent that some ofthemare equally applicable to other apparatus within the scope of thefollowing claims.

What is claimed is: i

1. In an .audiphone system, a pick-up microphone, a vacuum tubeamplifier having input and output circuits, a telephone receiverconnected to the output circuit, a volume control comprising a variablenegative feedback path within the amplifier for progressively improvingthe transmission characteristic of the amplifier as its gain is reducedand a network connecting the microphone to the input circuit, saidnetwork having an adjustabl low frequency attenuation whereby the lowfrequency output of the system may be connected to the output circuitofthe m tube, a path conductive to both signal and direct current fromthe output circuit of the last tube to the output circuit or the secondtube.a screen grid in the first tube and a second path conducting bothsignal and direct current from the output circuit oi the last tube tosaid screen grid.

4. An amplifier comprising a plurality of vacuum tubes each having aninput and an output circuit connected in tandem, a battery forenergizing the tubes and a negative feedback path extending from theoutput circuit of the last tube to the input circuit of one tube andconducting current from the battery to the output circuit of anothertube. I

5. In an audiphone system, the combination with a pick-up microphonehaving a predetermined capacity, a receiver, an amplifier connecting themicrophone to the receiver and a source of current for the amplifier, ofa resistance-ca- I pacity network interpoaed'between the micro? phoneand the amplifier and forming with the capacity of the microphone afilter of at least two sections for selectively attenuating lowfrequency currents.

8. An audiphone system according to the preceding claim in which theimpedances of the secone section of the filter are high as compared withthose of the first section whereby the variations in the impedance ofthe microphone do not affect the impedance of the second section of thefilter.

I. In an audiphone system, the combination with a crystal typemicrophone, a receiver and an amplifier having an input circuitconnected to the microphone and an output circuit connected to thereceiver, of a condenser serially connected to the other side of theinput circuit and means for modifying the efi'ect of the resistance onthe input circuit.

' 8. In an audiphone, the combination with a crystal yp pick-upmicrophone, a receiver and a vacuum tube amplifier connecting themicrophone to the receiver and having a substantially uniformtransmission characteristic, of a network for adjusting the response ofthe audiphone over a limited range of low frequencies without aiIectingthe transmission of other frequencies connected between the microphoneand the amplifier and comprising a series condenser, a resistance,by-passing the condenser, a resistive shunt arm connectedto anintermediate point on the resistance. and means for adjusting theeffective resistance of the shunt arm.

9. In an amplifying system, the combination with a vacuum tube amplifierhaving input and output circuits, a pick-up microphone connected to theinput circuit and an output transformer having a secondary winding and aprimary winding connected across the output circuit, .of a. receiverhaving a capacitative impedance connected across the output circuit anda resistor connected across the secondary winding to form with thetransformer a terminating network for developing across the receiver ahigh voltage which is substantially uniform over the frequency bandtransmitted by the amplifier.

10. In an amplifying system, an amplifier having input and outputcircuits, a pick-up microphone connected to the input circuit and atransformer having a secondary winding and a primary winding connectedto the output circuit.

CARL H. RUMPEL.

